Many applications require the use of a transportable antenna system. In such applications, it is frequently desirable to utilize an antenna that is capable of being folded into a compact volume for transport and later deployed into an extended operating configuration. As a result, many attempts have been made to design collapsible antenna systems of various kinds. Several such antenna designs are described in the copending U.S. patent applications, entitled "Self-Deploying Helical Structure", Ser. No. 08/192,324, now abandoned and "Axially Arrayed Helical Antenna", Ser. No. 08/191,247, both filed Feb. 4, 1994, and assigned to the assignee of the present invention.
An advantage found in some of these prior antenna systems is that the antenna can be stowed and redeployed without the need to disconnect and reconnect the antenna to its signal receiver/transmitter circuits. Typically, these antenna systems collapse and expand longitudinally toward and away from the base of the antenna where the connections to the feed lines, which connect the antenna to the signal circuits, are found. As a result, the feed lines are largely unaffected when the antenna system is stowed or redeployed.
In many applications, however, an antenna system having feed lines that are capable of contraction and expansion is found to be desirable. For example, in satellite communication applications it is desirable that the antenna system be collapsible about its entire radius to occupy as little volume as possible while the antenna is being transported to minimize the volume of spacecraft payload. In such applications, it is also desirable that the antenna redeploy to an operating configuration automatically with connections to receiver/transmitter circuits maintained. The above-referenced co-pending applications describe such a self-deployable antenna system that can be collapsed radially into a compact volume. However, because the connections to the signal circuits are also subject to movement and stress in these designs, self-deployable feed lines are required.
The materials and structure typically used as feed lines have disadvantages when used with a deployable antenna system. For example, when ferrous metals are used in a feed line, oxidation can occur at the metal-to-metal junction between the feed line and the antenna element, which can create a phenomenon known as passive intermodulation ("PIM"). PIM can result in garbled data transmissions over the frequency spectrum of the transmitted signal.
Moreover, non-ferrous metals, such as a copper wire segment, also have significant disadvantages when used as a feed line. Although somewhat resilient, copper wire segments fail to retain their shape when the antenna to which they are connected is collapsed and expanded during stowage and redeployment. When this occurs, adjacent feed lines in a multiple conductor antenna system can become entangled, which interferes with antenna redeployment.
Where an antenna element has multiple conductors, such as in a quadrifilar helical antenna, to provide a proper phase progression and amplitude among the conductors the feed lines must provide an identical signal path from the signal circuits to each antenna conductor. When, for example, the individual conductors are fed radially, each line must lie in the same plane if the antenna is to perform properly. Because, as noted above, copper wire and other materials typically used as feed lines fail to retain their original shape and spatial orientation when the antenna to which they are connected redeploys after stowage in a folded configuration, the individual feed lines of a multiple-conductor antenna element can be out of plane with respect to each other. As a result, the feed lines fail to provide identical signal paths causing degradation of the RF signal pattern of the antenna system.
Although a more flexible feed line can be made of copper wire--or another non-ferrous conductor--formed into a spring through heat-treating, this construction tends to be too stiff for the lightweight antenna systems that are desirable in spacecraft applications. As a result, the feed springs resist redeployment when the antenna system expands into its operating configuration. In addition, the heat-treated copper material is prone to fatigue and premature damage caused by bending stresses exerted on the feed spring when the antenna system is stowed and redeployed.
Therefore, it would be desirable to provide an antenna system having highly resilient and durable feed lines that are deployable from a compact stowed configuration to an extended operating configuration while maintaining their original shape and spatial orientation.